Multi-chassis link aggregation (MLAG) is the ability of two or more network elements to act like a single network element when forming link bundles. This allows a host to uplink to two switches for physical diversity, while still having a single bundle interface to manage. In addition, two network elements can connect to two other network element using MLAG, with all links forwarding.
In an MLAG topology, if there is switch A as primary and switch B as secondary and the user reboots switch A, this reboot triggers a failover to switch B, with switch B assuming the role of primary. After switch A comes back up online, switch A starts MLAG negotiation where it takes the role as secondary. As switch A boots up, the switch A ports (except for a peer-link port with switch B) start in an error disabled state during a delay in the newly formed MLAG peer. This allows that the required states are synced between MLAG peers and further allows a graceful entry for switch-A into the network as an MLAG peer.
After the delay expires, the ports in switch A change its status to UP. If switch A includes routing capability, switch A uses address resolution protocol (ARP) to resolve Internet Protocol (IP) addresses to media access control (MAC) addresses of device coupled to switch A. However, the ARP table will be empty and will require ARP resolutions for the unknown hosts. Since ARP cache is not persisted/synced between the peers, there can be traffic drops till the destination is resolved.